Thank you all for your thoughts and good wishes. I just heard back from my mother - apparently, Baron's blood tests (they did the full senior cat blood tests) came back normal, and the vet thinks that it's just his teeth that were bothering him, and that's why he wasn't eating and why he was clicking his teeth. They seem to be doing better now, and Mom gave him some baby food that he ate right up. He seems to be back to his normal self a bit.

Mom's going to be watching for the next few days to try and decide if it's worth the risk to put him under and get his teeth cleaned or possibly pulled. He has been pretty shaky lately, and I think she's just worried that he isn't strong enough to handle being under right now. Either way, she's going to keep an eye out, but she's just relieve it isn't something major.